1. Field of the Invention
The present invention relates to coded radio frequency tag identification systems and more particularly sorting through and collecting colliding reports caused by turning all the identity tags on at the same time in a small area.
2. Description of Related Art
Electronic item identification systems are in widespread use in a variety of applications. One familiar electronic item identification system uses bar code labels on retail shelf merchandise. These types of systems are typically used by supermarkets, distributors, shipping services and clothing retailers to scan the bar code labels for quick retrieval of an item's price or other information. Bar code labels can be printed by ordinary means as a series of lines of varying widths or thickness. The pattern establishes a code which can be read by a scanner, e.g., one based on a laser. The data from the scanner is electronically fed to a computer which interprets the bar code label into an identification code or number that is used to index a database of item price or other descriptors. The computer sends back the price and description to the cash register for checkout.
According to U.S. Pat. No. 5,444,223, issued Aug. 22, 1995, to Blama, another class of electronic item identification systems uses radio frequency (RF) identity tags to identify items. Such RF identity tags are used to identify a variety of items to which the tags are attached or otherwise associated, e.g., to identify passengers, luggage, library books, inventory items and other articles. RF identity tags also allow the electronic identification of people or objects, moving or stationary, at distances of several feet. In recent years, radio frequency identity tags have been manufactured using microminiature silicon chips. But, according to Blama, such chips are very expensive and cannot be produced in the quantities necessary to make the tags feasible. He contends that the silicon chip identity tags have a limited range of approximately two feet, e.g., using scanners that send out one signal that the chip phase shifts back. Blama says it would be desirable to develop a radio frequency identity tag that could be manufactured in mass quantities on a less expensive material than silicon such as paper or plastic and that could be used without having to alter the circuits on the tags.
Other types of RF identity tags are configured for recognition and surveillance functions. It would be desirable for RF identification cards to allow both recognition and identification functions with the card and on an inexpensive material. Interrogation and identification functions are not directly derivative from recognition and surveillance functions. For example, U.S. Pat. No. 4,694,283 to Reeb, and U.S. Pat. No. 4,910,499 to Benge, et al., each describe multilayered RF identity tags, wherein conductive layers are separated by a layer of dielectric material to form a resonant circuit. However, the Reeb patent limits itself to an electronic surveillance device, and the Benge, et al., is limited to an anti-theft device. These describe only recognition functions and do not teach the means necessary for identification. The recognition function can depend on reading the resonant frequency of the tag to find a match or on the return of some serial response code.
Some prior art RF identification cards use a pattern of binary ones and zeros in a code. A resonant circuit is alternated between a first and second resonating frequency. For example, U.S. Pat. No. 5,218,189, to Hutchinson, discloses a binary encoded multiple frequency RF identity tag and U.S. Pat. No. 5,103,210, to Rode, et al., discloses an security tag that can be activated and de-activated. Such tags both include an inductance connected in parallel with a capacitance comprising many individual capacitors which each have a predetermined different capacitance and which are connected in series. Rode, et al., teaches two capacitance branches that each have a predetermined capacitance with individual capacitors of each branch connected in series. The binary number codes are generated by switching the capacitors in and out. It would be desirable to develop an electronic item identification system using an RF identity tag wherein each circuit on the tag has a constant inductance and capacitance and thus the circuit itself does not have to be changed to check for the resonating frequency.
The devices of the Hutchinson and Rode, et al., patents short out capacitors during interrogation, and thus the circuit can never be restored to its original frequency to be read over again. It would be desirable to develop an electronic item identification system in which the RF identity tag can be read any number of times while still generating the same binary number as was read the first time, and in this manner the tag can be reused.
The Hutchinson and Rode, et al., patents teach a device where the binary number to be obtained from the tag must be predetermined. This is because the device teaches the "dimpling" of capacitors which, to be accurate, must be done with expensive, precision equipment. It would be desirable to develop an electronic item identification system in which an RF identity tag having numerous circuits made up of capacitor/inductor coil pairs at evenly spaced intervals on the surface of the tag so that the presence or absence of a circuit or the circuit's functionality could be programmed at the point of use with inexpensive equipment.
In U.S. Pat. No. 5,166,676, issued Nov. 24, 1992, Milheiser lists prior art identification device systems as usually having an exciter associated with an interrogator that is used to feed a combined alternating current clock signal and power supply signal to a responder device over an inductive coupling. The responders are implanted in animals or in other things with an identity that is to be ascertained, such as a freight car. The responder issues a coded identification signal which is fed back through the inductive coupling to a detector and demodulator to produce an output code representing the particular animal or thing being identified.
Many coded data transmission systems have been used. In some, the responder comprises a resonant circuit which varies in frequency according to the identification code. In Kaplan, et al., U.S. Pat. No. 3,689,885, coded information is returned from a responder to an interrogator as fixed-frequency CW bursts. In Beigel, U.S. Pat. No. 4,333,072, the responder or tag circuit produces a signal by varying the load across the inductor responsive to the encoded signal characteristic of the animal or thing being identified. All of these systems are subject to certain drawbacks, e.g., the resonant circuit systems are susceptible to temperature variations that affect the resonance frequency. Such variations produce spurious frequencies that are difficult to guard against. Systems that use code signals suffered from variations in amplitude of an oscillating circuit.
The identifying device may take the shape of a credit card having an electronic circuit embedded therein for radiating signals of identifying intelligence. An individual possessing the card may position it adjacent to a door that they want to enter. A recognition device may be arranged to control the door latch. Thus, if it recognizes radiated signals of certain predetermined intelligence, the door latch responds to the reader when the individual places the "card" proximate to the reader. Other uses for such systems include having the identifier in the form of a tag attached to a vehicle to be identified. Also, in production lines, garments or items may carry identifying tags so that they can be appropriately processed as they are recognized along various points in the production processes.
In the electronic portable recognition and identifier systems of the prior art, there are two directions of communications between the reader and the identifier devices. A stationary reader sends an interrogation signal and/or power to a portable identifier device, e.g., card, tag, key. In response, the identifier device sends a coded identification signal back to the reader. Means must be provided so that the two directions of communication and power do not inhibit one another. In the general class of electronic portable identification and recognition systems, inductive coupling is used between the reader and the identifier, as contrasted with electric field dipole antenna coupling systems. In conventional systems, often both power and data are transmitted over the same inductively coupled coils. In some, interference between the two paths of the reader and identifier is avoided by using time or frequency separation, or by modulating the electromagnetic power field and detecting such modulation in the radiated reader field. But time separation increases the total transaction time. Frequency separation means the coil of the portable identifier cannot be tuned simultaneously to two different frequencies. Such inability to tune, makes one direction inefficient. According to Walton, a disadvantage of power field modulation is that power losses occur in the identifier when the identifier modulates the power field.
None of the prior art appears to address the problem of sorting through many identity tags that are attempting to report-in simultaneously. Each teaching concerns itself with the basic communication mode; and given the crowded appearance of the subject area, there are several mediums to pick from that can now be considered conventional.